Information - Assassin
"A mysterious girl who appeared suddenly before Felt's eyes one day. Her appearance is cold, and she wields two large crimson scythes." - Official Site
"A mysterious girl who attacked Felt out of nowhere. It seemed she had a purpose for the assault. But eventually the misunderstanding is removed, and she becomes a part of Felt's group." - Game Manual
"Fee is an orphan. As an infant, she was found at the steps of the Altena Church, where the high priestess Eizlen raised her. As an adult, Fee serves the church as a holy warrior. She wields twin scythes and is sent out to destroy that which would threaten her religion. At first, unfortunately, that includes Felt." - Prima Strategy Guide
You first encounter the mysterious woman in Episode 4: Infiltrating Riesevelt, as a boss duel. She goes on about how the Azoth must be destroyed, but after you defeat her she isn't heard of again until Episode 6: Assassin in White. While traveling through the Sessier Forest, you notice that she is succumbing to the local wildlife which consists of poisonous creatures.
While you manage to fend off the creatures, she is poisoned. After taking her back to Max's camp, the group deduces that the only way to save her is by synthesizing a Cure Jar. Unfortunately, they have no idea how. Fortunately, there's a peddler out there with the antidote recipe, and with that Viese was able to synthesize a Cure Jar and rescue the assassin's life.
After she recovers, you learn that she is Fee, a holy warrior of the Church of Altena. She mentions that she is seeking to destroy the Crimson Azoth; however, this misunderstanding quickly manages to calm her down as she learns that Felt's Azoth is the Azure Azoth. The group and Fee decides to go to Altena Church to let her superior decide.
On the way to the church, Felt and Fee run into Chaos, and are prompty knocked out. Who knows what else would have happened if Gray hadn't intervened? The two were safe, however.
Upon arriving at the church, the High Priestess Eizlen and Yuveria clear up the misunderstanding, that it is the Crimson Azoth which must be destroyed.
Afterwards, we learn that Fee is an orphan, who was left on the steps of the church twelve years ago. High Priestess Eizlen raised her as an ordinary orphan, and Fee dedicated her life to serving the church that raised her. After learning more about the nature of Eden, Eizlen asks Felt if Fee may accompany him, stating that she was still innocent of the outside world. Felt happily agrees.